1. Field of the Invention
The present invention relates to corrosion detectors, and more particularly to corrosion detectors for pipelines.
2. General Background
Corrosion of pipelines occurs on the exterior of pipelines and on the interior.
Corrosion of the interior of the pipeline most commonly occurs, in a pipeline carrying gas or liquid condensate, at the bottom of the pipe, where water which condenses out of the gas, or separates from the oil, settles. This water causes the pipe to corrode.
Corrosion of the interior of the pipe also occurs at other locations, such as the "12 O'clock" position (the uppermost part of the interior of the pipe). "12 O'clock" corrosion can occur when a pipeline carrying a liquid is not completely full. Water condenses, out of gas located above the liquid, onto the upper part of the pipe.
Corrosion of pipelines, if undetected, can be a serious problem. If the pipeline carries natural gas, and the pipeline explodes due to corrosion, the results can be disastrous. Also, if the pipeline is carrying petroleum, unchecked corrosion can result in leaking oil contaminating the environment.
To prevent corrosion of the exterior of the pipelines, pipelines are usually painted and/or are attached to a piece of metal having a lower electric potential than the pipeline. The piece of metal acts as a sacrificial anode, accepting free electrons from the pipeline and corroding so that the exterior of the pipeline does not corrode. However, the protection of this anode does not extend through the wall of the pipeline to prevent the pipeline from corroding on its interior.
There is presently no system for effectively preventing corrosion of the interior of pipelines. Sacrificial anodes are not used inside pipelines because they would impede flow of fluid through the pipelines and also because they would get in the way of pigs (in-line scrapers) used to clean the interior of the pipelines. There are, however, a couple of systems which attempt to detect corrosion on the interior of pipelines.
One system utilizes a coupon (a flat, thin sheet of metal) placed inside the pipeline at the "12 O'clock" position. Periodically, the coupon is removed from the pipeline and weighed or otherwise measured to determined how much it (and presumable the pipeline) has corroded.
Another corrosion detection system presently in use is a linear polarization probe system. That system comprises placing a thin wire in the pipeline. A meter is connected across the ends of the wire. As the wire corrodes, the resistivity of the wire increases, and this increased resistivity is measured by the meter.
While the two detection systems sound useful in theory, in practice their application is limited to points at which the pipeline is above ground. Further, if corrosion is taking place at the bottom of the pipeline, but not at the top, these systems may not detect it. Moreover, corrosion may take place underground. If the systems are only used on above-ground sections of pipeline, they will likely fail to detect corrosion occurring in underground sections of the pipeline.